This disclosure is directed to a borehole spectral analog to digital converter or spectrum analyzer. It is a device especially suited for use with a photomultiplier tube functioning as a scintillation pulse detector. The system converts photomultiplier tube analog pulse outputs into digital representations of such pulses. These digital representations have the form of numbers that represent the energy of gamma rays or other nuclear radiations causing scintillations in a detector crystal optically coupled to the PMT. Other techniques have been used heretofore and some of their limitations are noted. One approach used in the past is the Wilkinson ramp technique in which the PMT pulses are integrated onto a capacitor which controls a counter during discharge. The counter output is the digitized value. Another approach used heretofore is the peak detector approach in which the highest voltage pulse transient is sensed and stored in a sample and hold gate. An analog to digital converter digitizes this temporarily stored value. The peak detection approach utilizes a fixed conversion time and is deemed to be relatively complex and especially sensitive to temperature drift. The Wilkinson ramp technique has a variable conversion time which depends on pulse amplitude and is also temperature sensitive. A number of errors arise as a result of temperature drift including variations in gain, offset, and other nonlinear errors in the integration.
Various devices have been provided heretofore including U.S. Pat. No. 4,042,824. That is a dual system spectral analog to digital converter. As shown in FIG. 1 of that disclosure, radiation is detected and the pulses are stretched, and then input for digitizing. The digital value is stored in a register. Among other things, the performance of that system is sensitive to the adjustment of the pulse stretcher. U.S. Pat. No. 3,421,093 discloses a base line correction system utilizing a feedback loop with sample and hold circuits in it. It is intended primarily for base line correction. U.S. Pat. No. 3,192,371 discloses a reversible counter and a digitizer cooperative with the counter. It is a system intended primarily for proportional digitizing in an integrating system. U.S. Pat. No. 3,765,012 discloses a reversible counter in a feedback loop cooperative with an integrator. By and large, these devices do not provide the advantages which are found in the disclosed apparatus. This disclosure sets forth a borehole scintillation digitizer. Temperature drift inevitably encountered in severe conditions in a down hole sonde is readily accommodated. The drift is inevitable as the device is lowered deeper and deeper into a borehole. The system incorporates a feedback loop which encounters the drift arising from noise, voltage, error integration and forms a feedback signal to cancel this error.
This system converts the output signal of the photomultiplier tube into proportional voltage pulses which are summed at an integrating amplifier. They are summed to form a DC value which is proportional to the charge from the photomultiplier tube which is integrated over a period of time. This assists in converting the incident gamma ray on the photomultiplier tube into a proportional value. This is dynamic over a period of time and is, therefore, more representative of gamma ray energy impingement on the photomultiplier tube in contrast with simple peak detection. Moreover, the feedback system which is incorporated not only compensates for temperature induced errors but it also overcomes noise generated at the photomultiplier tube (PMT hereafter). The feedback loop thus corrects for such errors and restores the output signal to the correct integral sum over a time period of PMT output. This value is digitized and transmitted to the surface from the sonde.
This disclosure not only sets forth apparatus but also describes a method by which the signal from the PMT is integrated and digitized. It contemplates integration of the PMT output signal over a defined time interval. This input analog scintillation signal, after integration, is periodically sampled to obtain a peak value which is representative of the sum. This peak value is digitized to form a digital value or word for easy transmission to the surface. Moreover, this procedure forms an offset signal to cancel noise, voltage, and drift arising from temperature drift. All errors from multiple sources are summed with the input signal in advance of integration to offset cumulative errors recognized in the input signal typically arising from noise or temperature drift.